The present invention deals with a method for manufacturing reactive injection-molded polyurethane products.
In general, a method for manufacturing reactive injection-molded polyurethane products is characterized by the following: highly chemically active liquid ingredients are mixed together in a mixing chamber under pressure, and the resulting mixture is injected into a sealed mold in which the mixture is allowed to set as a reaction takes place in the mixture.
Due to the fact that highly chemically active individual liquid ingredients are injected into a sealed mold as soon as they are mixed together, and the mixture is allowed to set as a reaction takes place in the mixture, the present method offers the following advantages: the mixture has good flow properties which in turn makes it possible to manufacture products in various shapes and with good appearance and to shorten the manufacturing time. Because of these advantages, the method has been used to manufacture, for example, furniture, shoe soles and heels, rollers, gears, bearings and other industrial parts, bumpers, fenders, fascia and other automobile parts, housings for home appliances, audio equipment, and so forth.
However, the liquid urethane used in the process is too reactive and tends to bond to the molds used. As a result, it is necessary to apply external mold-releasing agents to the molds at certain time intervals. The problem of continuously applying mold-releasing agents is one of the major obstacles that has to be overcome before the manufacturing time can be shortened significantly. In general, external mold-releasing agents are usually mixed solutions of wax or soap and low-boiling organic solvents. When applied on molds, such mold-releasing agents contaminate the manufacturing processes with the organic solvents they contain. Furthermore, molded polyurethane resin products are usually painted and thus external mold-releasing agents must be washed away before painting and thus, extra time is added to the painting process.
To eliminate such problems, the use of organosiloxanes, containing carboxyl groups, as internal mold-releasing agents has been proposed in U.S. Pat. No. 4,220,727, issued Sept. 2, 1980 and U.S. Pat. No. 4,076,695, issued Feb. 28, 1978. However, the carboxyl groups found in such agents tend to interfere with the setting of urethane resins and, in some extreme cases, the urethane resins do not even set. To eliminate this problem, it is necessary to significantly increase the amount of catalyst added to the resin. Unfortunately, polyurethane resin products manufactured using such large quantities of catalysts have physical properties that are significantly inferior to those of polyurethane products manufactured under ordinary conditions. In particular, the weatherability of such polyurethane products is markedly inferior to that of polyurethane molded products manufactured under ordinary conditions.
Furthermore, as it is widely known in the industry, that in the actual reactive injection-molding processes, it is most common to mix organoisocyanates with premixes (polyols, catalysts, chain-extending agents, etc.) under pressure and, in general, the molding machines have two supply tanks containing these starting materials. However, the carboxyfunctionalsiloxanes of the prior art have been found to be co-reactive with the ingredients in these supply tanks and therefore, the carboxyfunctionalsiloxanes cannot be stored in the organoisocyanate tank nor in the premix tank. In other words, it is necessary to install a separate tank to store the organopolysiloxanes.
The present inventors have conducted extensive research to eliminate the aforementioned defects of the conventional method for manufacturing products by reactive injection molding using conventional mold-releasing agents and achieved the present invention.